A separation membrane such as microfiltration membrane and ultrafiltration membrane is utilized in various fields including food industry, medical treatment, city water production, wastewater treatment and other fields. In particular, a separation membrane is recently used also in the field of drinking water production, i.e., in the water purification. As to the separation membrane used for a water treatment such as drinking water production, the amount of water that must be treated is large, and excellent water permeation performance is therefore required.
Furthermore, the separation membrane is preferably resistant to clogging, i.e., capable of continuously maintaining high water permeation performance. By using a membrane having excellent water permeation performance, the same desalination amount can be realized with a smaller membrane area, so that the water treatment apparatus can be made compact and consequently, the equipment cost can be saved. In addition, the water treatment apparatus equipped with a membrane resistant to clogging can be operated at a low cleaning frequency with a low power consumption and therefore, is advantageous from the viewpoint of operating cost as well.
The separation membrane is sometimes put into contact with a sterilizer such as sodium hypochlorite used for sterilizing permeate or preventing biofouling of the separation membrane. Furthermore, the separation membrane is sometimes washed with an acid, an alkali, chlorine, a surfactant, etc. Accordingly, the separation membrane preferably has chemical resistance.
In recent years, there is an emerging problem that pathogenic microorganisms resistant to chlorine (for example, cryptosporidium derived from livestock excreta, etc.) get mixed in with clean water due to damage to the separation membrane.
That is, the separation membrane is required to have sufficient separation properties and high strength and elongation.
As to the conventional production method for a separation membrane, for example, Patent Document 1 discloses a wet solution method where a polymer solution prepared by dissolving a polyvinylidene fluoride-based resin as a thermoplastic resin in a good solvent is molded at a temperature fairly lower than the melting point of the polyvinylidene fluoride-based resin and then brought into contact with a liquid containing a nonsolvent for the polyvinylidene fluoride-based resin to cause nonsolvent-induced phase separation to thereby form an asymmetric porous structure.
In more recent times, as disclosed in Patent Document 2, there is a melt-extraction method where a polyvinylidene fluoride-based resin as a thermoplastic resin is melted and kneaded with an inorganic particle and an organic liquid, the mixture is extruded and solidified at a temperature not less than the melting point of the polyvinylidene fluoride-based resin, and the organic liquid and the inorganic particle are then extracted to form a porous structure.
Patent Document 3 discloses a method where a polyvinylidene fluoride-based resin solution containing a polyvinylidene fluoride-based resin as a thermoplastic resin and a poor solvent for the resin and having a temperature not less than the phase separation temperature is solidified in a cooling bath at a temperature not more than the phase separation temperature to cause thermally induced phase separation to thereby obtain a separation membrane having a spherical structure of 0.3 μm to 30 μm and having relatively high strength/elongation.